This invention relates to an optical data processor for recording and reading out data for optical discs by using a focused beam.
Data filing systems for filing a tremendous number of documents, and drawings, have been developed and put to practical use. In the system, the data on the documents are optically and two-dimensionally scanned, and collected in the form of electrical data. The data is recorded into an image recorder. The recorded data is retrieved, and reproduced in the form of hardcopy or softcopy, if necessary. In the filing system of this type, an optical disc device (optical information processor) has recently been used for such an image recorder.
The optical disc device uses an optical disc for recording the data in a spiral fashion. In recording or reproducing the data, an optical head is driven by a linear motor and rectilinearly moved in the radial direction of the disc.
In the optical disc device, an error is inevitable in mounting an objective to the optical head, and causes the optical beam to inaccurately land at the target position on the optical disc when focusing or tracking of the objective is performed. To cope with this, the difference between two types of detect signals for focusing is calculated. The current based on the difference signal is fed through a voice coil for driving the objective. The objective is driven to the correct focal position. Similarly, a difference between two types of detect signals for tracking is calculated. The current based on the difference signal is fed to the objective drive coil, to move the objective to a correct tracking position.
The focusing and tracking controls must be stable against the detect level difference of the reflecting signal from the optical disc in the record and reproduction mode. To obtain stable control, a divider for normalizing the drive signal (for dividing the difference of the two different detect signals by the sum of them) is used. Additionally, a measure has been taken to eliminate physical error such as caused by the sensitivity error of detectors and the error due to the incorrect position of the objective in its mounting. In this measure, the offset correcting voltage is added to the drive signal.
In the optical disc device as mentioned above, however, the two detect signals, when the optical filing system is in the record mode, are much larger than those when in the read mode, and frequently reach values several times the latter. The difference of these signals between the record and read modes has little influence on the difference between the two detect signals, but greatly influences the sum of them. In the extreme case, the sum in the record mode is several times that in the read mode. If the gain of the system is set such that the sum signal is not saturated at the power voltage, the sum signal is too small in the read mode so that the operation of the divider may be unstable. The divider, which operates stably against the too small sum signal, could be realized, but is expensive.
To cope with the great increase of the detected light amount in the record mode, the divider must be designed to have a wide dynamic range and a high accuracy of calculation. This represents a difficult application of the device and a high cost to manufacture the device.